With the heightening of environment consciousness in recent years, attention is focused on soil contamination problems caused by waste plastic products, and attention is also focused on global warming problems due to an increase in the carbon dioxide concentration caused by combustion of waste. Research and development studies have been performed actively on various biodegradable resins and biomass (plant-derived material) based resins, whose incineration will not increase the carbon dioxide load on the atmosphere, as means of solving the former and the latter problems, respectively. Expectations are growing for polylactic acid as it is able to meet both of the above purposes and relatively advantageous in terms of cost as well. Polylactic acid materials cannot be sufficiently high in flexibility and impact resistance if applied to uses where flexible films of polyolefins such as polyethylene have been conventionally adopted, and a variety of attempts are being made in an effort to improve them in these characteristics to provide practical materials.
In the field of porous film production, Japanese Unexamined Patent Publication (Kokai) No. 2007-112867, for instance, has disclosed a porous sheet that is produced by stretching at least uniaxially a sheet containing a polylactic acid resin, a filler, and a common polyester based plasticizer. Japanese Unexamined Patent Publication (Koaki) No. 2004-149679 has disclosed a porous film produced from a polylactic acid based polymer, aliphatic aromatic copolymerization polyester and, in addition, a common plasticizer selected from the group consisting of an aliphatic multivalent carboxylate, aliphatic polyhydric alcohol ester, aliphatic polyhydric alcohol ether, and oxyacid ester, in which pores are formed by adding a fine powder filler.
The aforementioned techniques described in JP '867 and JP '679 can produce biodegradable flexible films with a high biomass content, but they are inferior in processability.
Thus, although studies have been made aiming to provide flexible films with high biodegradability and high biomass content, they all have failed to have a sufficient moisture permeability, and no efforts have been successful in inventing such films also having both high moisture permeability and high bleed-out resistance.
It could therefore be helpful to provide a polylactic acid based film that has high flexibility, heat resistance, bleed-out resistance, and durability as well as high processability to permit stretching and embossing required for developing high moisture permeability.